Field of the Invention
The present invention relates to a hydrocarbon gas sensor that detects a hydrocarbon gas in a measurement gas.
Description of the Background Art
There has been already known an apparatus capable of measuring a total concentration of inflammable gases in a measurement gas containing, as inflammable gases, carbon monoxide (CO), hydrogen (H2), and hydrocarbon (HC), or the concentration of HC alone in the inflammable gases (for example, see Japanese Patent Publication No. 3450084). Combustion exhaust gases in, for example, an internal combustion engine, an external combustion engine, and a combustion path are illustrated as examples of this measurement gas.
In outline, the apparatus disclosed in Japanese Patent Publication No. 3450084 includes a first processing zone and a second processing zone each defined from the outside and is configured to apply, between an opening portion facing the outside and the first processing zone and between the first processing zone and the second processing zone, a predetermined diffusion resistance to a gas atmosphere flowing therebetween. In this apparatus, the measurement gas containing inflammable gas components is first guided to the first processing zone from the outside of the apparatus with a predetermined diffusion resistance, and then, its oxygen partial pressure is reduced such that the inflammable gas components are not substantially burned, through an oxygen pumping action by a first electrochemical oxygen pumping cell in the first processing zone. After that, the measurement gas is guided to the second processing zone with a predetermined diffusion resistance. Then, in the second processing zone, the inflammable gas components existing in the atmosphere are burned when being supplied with oxygen through the oxygen pumping action by a second electrochemical oxygen pumping cell. The pump current flowing through the second electrochemical oxygen pumping cell or the voltage generated between the electrodes of the second electrochemical oxygen pumping cell is detected in this burning, to thereby obtain a concentration of the inflammable gas in the measurement gas based on the detected value.
A selective oxidation catalyst, which does not oxidize a hydrocarbon gas but selectively oxidizes inflammable gases except for hydrocarbon gases, such as a carbon monoxide gas and a hydrogen gas, is placed between the opening portion and the first processing zone, so that the hydrocarbon gas alone is guided to the second processing zone As a result, the concentration of the hydrocarbon gas can be obtained.
In a case where the above-mentioned concentration measurement technique is performed with the apparatus disclosed in Japanese Patent Publication No. 3450084, the oxygen partial pressure of the measurement gas is controlled to such a low value as not to substantially burn the inflammable gas, and thus, the first processing zone has an extremely low oxygen concentration. Meanwhile, oxygen is supplied to the second processing zone to burn the inflammable gas, so that the second processing zone has an oxygen concentration much higher than that of the first processing zone. The first processing zone and the second processing zone are in communication with one another, and thus, such a difference in oxygen concentration between the processing zones causes oxygen to flow into the first processing zone having a lower oxygen concentration from the second processing zone having a higher oxygen concentration. On this occasion, oxygen is discharged to the outside of the element through the oxygen pumping action by the first electrochemical oxygen pumping cell in the first processing zone, whereby the oxygen that has flowed from the second processing zone is discharged to the outside of the element as well. This results in an insufficient amount of oxygen for burning the inflammable gas in the second processing zone, and accordingly, a larger amount of oxygen than required for burning the inflammable gas is supplied to the second processing zone.
In the above-mentioned concentration measurement technique, the concentration of the inflammable gas is obtained based on a pump current flowing through the second electrochemical oxygen pumping cell in oxygen supply or a voltage generated between the electrodes of the electrochemical oxygen pumping cell. However, a larger amount of oxygen than originally required for burning the inflammable gas is supplied, and accordingly, a concentration value of the inflammable gas is calculated as a value much larger than an original value.
Such a phenomenon becomes more conspicuous as an oxygen partial pressure in the first processing zone is set to a smaller value to increase measurement sensitivity, further increasing measurement errors.